The present invention relates generally to a method and apparatus for preventing leakage of electromagnetic interference, and more particularly, to a method and apparatus for capping and grounding an electrical connector to prevent leakage of electromagnetic interference. The present invention is of particular use in avionic instrumentation display units. Avionic instrumentation display units are essential on military, commercial, and private aircraft to provide the pilot and co-pilot, if any, with the information which is necessary to fly the aircraft. Typically, the primary avionic instrumentation display units are located on the instrumentation panel which is below the windshield and in front of the seats occupied by the pilot and co-pilot, if any. On the other hand, the secondary avionic instrumentation display units are often located on an overhead instrumentation panel that is above the windshield.
The majority of modem military, commercial, and private aircraft instrumentation panels utilize ARINC cutouts for the avionic instrumentation display units. In other words, there is a standard instrumentation panel cutout for each avionic instrumentation display unit. The ARINC cutouts are spaced closely together. The dimensions and close proximity of the ARINC cutouts limit the size of the face of each display unit. In addition, the dimensions and close proximity of the ARINC cutouts limit the chassis size of each display unit. A chassis commonly houses the circuitry and components of the display unit. Due to restricted chassis size, only a limited amount of space is available to mount and wire the circuitry and components of each display unit.
As a further result of the aforementioned size limitations, electromagnetic fields may interfere with the performance of avionic instrumentation display units. An electromagnetic field is a combination of electric and magnetic fields. Propagating electromagnetic fields are produced by charged particles that are subjected to acceleration. For example, the flow of electrons in a conductor carrying an alternating current produces an electromagnetic field that propagates outward from the conductor.
When an alternating current flows through a conductor, the resulting electromagnetic field exerts forces on nearby charged particles. In particular, the electromagnetic field causes electron movement at the same frequency in nearby conductors even though the nearby conductors are not electrically connected to the current-carrying conductor. This phenomenon is known as electromagnetic induction.
Electromagnetic interference is an electromagnetic field that interferes with other signals and degrades the performance of an electronic device. Three general methods of suppressing electromagnetic interference are grounding, shielding, and filtering. For instance, it is known to properly ground the signals in a system, and it is known to utilize electromagnetic shielding. It is also known to physically isolate the power from the other signals. This may be accomplished by physically separating the wiring and by physically separating the runs on the circuit boards. Furthermore, it is known to place a line filter in series with an alternating current power input in order to suppress electromagnetic interference to an acceptable level while permitting the 50 or 60 Hz current to pass with little or no attenuation. In this manner, a line filter essentially traps electromagnetic interference and prevents it from leaving.
Electromagnetic shielding prevents an electromagnetic field from entering or leaving a desired area. In other words, electromagnetic shielding prevents unwanted coupling between circuits. A common form of electromagnetic shielding generally includes a grounded chassis that is made of conductive material. As used herein, the term "conductive" shall be understood to mean electrically conductive. The grounded chassis serves as the reference point for an electronic circuit. The grounded chassis may be perforated. However, the apertures should be very small compared to the wavelength of the electromagnetic field in order to effectively block the electromagnetic field.
Penetrations in a chassis limit its shielding effectiveness. Nonetheless, a number of penetrations are typically made in a chassis for electronic equipment. For instance, a chassis for electronic equipment may have penetrations for convection cooling, forced air cooling, displays, fuses, switches, potentiometers, lamps, screws, windows, lights, filters, and electrical connectors. Consequently, the total shielding effectiveness is a function of all leakages associated with the various penetrations in the chassis.
A chassis for a piece of electronic equipment may have numerous penetrations for electrical connectors. Moreover, electromagnetic interference may enter or leave a chassis through any penetration for an electrical connector. An electrical connector may have a conductive shell that blocks some electromagnetic interference from entering or leaving the chassis through the penetration. However, a typical electrical connector also has a nonconductive insert that may allow electromagnetic interference to enter or leave the chassis through the penetration.
Consequently, a need exists for a method and apparatus that limits and preferably prevents leakage of electromagnetic interference into and out of a chassis through an electrical connector. Another need exists to cap and ground an electrical connector to prevent leakage of electromagnetic interference. Finally, there is still another need to cap and ground an electrical connector without consuming critical space within a chassis. As used herein, the term "cap" shall be understood to mean to position a piece of material relative to an electrical connector such that the piece of material may be adapted to block at least some electromagnetic interference from propagating through the electrical connector to a predetermined space.
The present invention satisfies one or more of these needs by providing a method and apparatus for capping and grounding an electrical connector. A preferred embodiment of the apparatus of the present invention includes an EMI plate that is adapted to prevent leakage of electromagnetic interference through a penetration in a chassis panel. The chassis panel is part of a chassis that houses a piece of electronic equipment. The chassis panel has a conductive area that may serve as chassis ground. The electrical connector is secured to the chassis panel, and it is disposed over the penetration. The electrical connector has a conductive shell that is electrically tied to chassis ground. The EMI plate caps the electrical connector, and it is also electrically tied to chassis ground. The EMI plate is preferably adapted to substantially prevent leakage of electromagnetic interference into and out of the chassis through the electrical connector.
A preferred embodiment of the EMI plate permits the transmission of desired signals into and out of a chassis via an electrical connector and associated conductors while substantially preventing electromagnetic leakage through the electrical connector. Accordingly, the EMI plate preferably has a plurality of conductor apertures. For example, the EMI plate may have a non-conductive aperture that is adapted to allow a predetermined conductor to pass through the EMI plate without being electrically tied to chassis ground. The EMI plate may also have a grounded aperture that is plated with conductive material that may be electrically tied to chassis ground. In addition, the EMI plate may have a conductive aperture that is plated with conductive material that is not electrically tied to chassis ground. For example, the EMI plate may form a portion of a printed wiring board, and the conductive aperture may be electrically tied to a run on the printed wiring board.
The electrical connector may be connected to the chassis by conventional means. The EMI plate may also be connected to the chassis by conventional means. It is preferred that the EMI plate is connected to the conductive shell of the electrical connector. In a preferred embodiment of the present invention, the EMI plate is secured to the conductive shell of the electrical connector by solder in bushings. In another preferred embodiment, the EMI plate is integrally connected to the shell of the electrical connector.
The EMI plate may be located inside or outside the chassis. The EMI plate is preferably located substantially adjacent to the rear of the electrical connector on the inside of the chassis. The EMI plate may simply cover the non-conductive insert of the electrical connector. However, it is preferred that the EMI plate substantially covers the penetration in the chassis as well as the non-conductive insert of the electrical connector.
The EMI plate may be comprised of any material that is adapted to block electromagnetic interference. The material may vary according to the particular application of the EMI plate. For instance, the EMI plate may be plated with a good conductor, such as copper or aluminum, in order to obtain high reflection loss of electromagnetic interference. On the other hand, the EMI plate may be plated with a relatively poor conductor, such as iron or a high-permeability alloy, in order to obtain high absorption loss of electromagnetic interference.
In a preferred embodiment of the present invention, approximately the entire EMI plate is plated with conductive material except in the immediate vicinity of any portion that is not desired to be conductive. Accordingly, it is preferred that the conductor apertures of the EMI plate are defined by non-conductive material except for those apertures which are desired to be conductive. In addition, it is preferred that the conductive material is sufficiently thick to provide adequate shielding against electromagnetic interference within a predetermined range of frequencies.
For applications that require a higher degree of shielding, at least one additional electromagnetic shield may be utilized which is adapted to further prevent leakage of electromagnetic interference into and out of the chassis through the electrical connector. In embodiments that utilize at least one additional electromagnetic shield, the additional electromagnetic shield(s) may be electrically tied to chassis ground. Similar to the EMI plate, the additional electromagnetic shield(s) may be comprised of or plated with any material that is adapted to block electromagnetic interference.
The additional electromagnetic shield(s) may form part of a printed wiring board. Like the EMI plate, the additional electromagnetic shield(s) may have one or more non-conductive apertures, conductive apertures, and/or grounded apertures. Moreover, each of the aforementioned apertures is preferably adapted to receive a respective conductor that passes through an aperture in the EMI plate.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments.